Making refractory shell molds



3,048,905 MAKING REFRACTORY SHELL MOLDS Charles L. Benson, Goshen, N.Y., and Stanley C. Tingquist, Sparta, NJ, assignors to Howe Sound Company, New York, N.Y., a corporation of Delaware No Drawing. Filed June 11, 1959, Ser. No. 819,573 15 Claims. (Cl. 22-193) This invention relates to refractory shell molds and is directed particularly to the provision of an improved method of making such molds. By the method of the invention it is possible to produce shell molds approaching in accuracy and substantially equal in surface quality to molds made by the lost wax process of precision casting, but at a much lower cost. This result is achieved by forming half-mold shells on match plate patterns by a technique which permits stripping the refractory shells without damage from the match plate, using a' hardenable refractory composition analogous to those employed in making shell molds by the lost Wax process. The half-mold shells thus produced are then assembled together to make the complete casting mold.

Precision castings, particularly of high melting point etals, heretofore have been most commonly made by an investment casting process or some other variant of the ancient lost Wax procedure. In general this involves making a pattern of the desired casting in wax or other easily fusible or volatile metal. The pattern is coated with or otherwise enclosed in a slurry of a hardenable refractory composition. After such coating has hardened, and has been sufficiently reinforced to withstand the thermal and casting stresses to which it must be subjected, the pattern is melted and run out or otherwise eliminated from within the body of refractory. Thus a refractory mold having a cavity conforming to the shape of the pattern is formed. The method lends itself very well to the use of refractory compositions which form very smooth mold surfaces and so make possible the production of castings having correspondingly fine smooth surfaces. It also makes possible the use of mold-making techniques by which accurate dimensional control of the casting is achieved. For these reasons this mold-making procedure has been extensively developed and used for making high quality castings of the more refractory metals, especially dental castings, jewelry castings, and industrial castings requiring a notably high degree of perfection in surface condition and dimensional accuracy.

Precision casting in refractory molds has heretofore been limited to making only rather small castings, and to making castings for which a relatively high manufacturing cost is justified. The size limitation is imposed by the technical dii'liculties of producing satisfactory molds from large wax or equivalent patterns; and the relatively high manufacturing cost results from the need for making and eliminating a pattern for each casting, and from the careful attention that must be given to the steps of forming the refractory mold on such patterns and eliminating the pattern after the mold has been formed. Moreover, the individual attention that must be given to each mold throughout the steps of forming it and eliminating the pattern prevents adapting it to lowcost mass production methods for making large numbers of identical castings. It has long been recognized that there would be widespread use for a relatively lowcost process for making refractory casting molds in which castings rivalling precision castings in surface quality and dimensional accuracy could be made, if such process could be operated successfully in commercial foundry operations for producing molds at a high rate of output with but limited use of skilled personnel. Heretofore, however, no such process has been developed.

States atent The present invention provides a method of making refractory casting molds which does substantialiy achieve these desired advantages. Refractory casting molds made by the new method possess a surface quality which may be equal to precision casting molds heretofore made, and they are adapted to making castings approaching precision castings in dimensional accuracy. The new method is much more economical than heretofore known precision casting techniques for making large numbers of identical castings; and it is well suited to use in largescale foundry operations with less labor, and less skilled labor, than is required for making precision casting molds by the lost wax process or any equivalent process.

The new method of making refractory casting molds comprises forming a pair of mating half-mold shells by applying a coating of hardenable liquid refractory composition to patterns on a match plate, and embedding in the surface of such coating while it is still fluid a layer of granular refractory particles. The resulting coating is allowed to harden on the match plate, and successive coatings of hardenable refractory composition are similarly applied until a refractory shell of substantial thickness has been built up on the match plate. When the half-mold shell thus formed has been hardened sufficiently, it is stripped from the match plate. After stripping, the shell is heated to a high enough temperature to fully set the refractory composition.

Half-mold shells produced in this fashion are assembled together to form the complete casting mold. (The term half-mold is used throughout this specification to mean shells defining only a part of a mold on one side of a parting plane. Such shells may form more or less than exactly half of the mold; and moreover three or more mating shells, instead of just two, may be required to form the complete mold.) The half-molds may be assembled directly after stripping from the match plate and before being heated to set the refractory, or they may be assembled after such heating. In either case, and especially if assembled before heating to the setting temperature, the assembled mold may be sealed by applying to its exterior (especially over the parting line) a coating of hardenable liquid refractory composition. The sealing composition is then hardened on the assembled mold and preferably is then heated to a temperature at which it fully sets.

A most convenient procedure for making the half-mold shells is to flow excess of the hardenable coating composition over the match plate and then drain excess away, as by inverting the match plate. Similarly the dry granular refractory may be distributed in liberal excess over the surface of the freshly applied coating of liquid refractory composition, and then the excess which does not become embedded in and adherent to the coating may be removed, as by inverting the match plate to dump the excess off.

In order to attain the maximum benefit of the new method for achieving a high output of refractory molds, it is desirable to accelerate hardening of the refractory liquid composition after each successive coat is applied, and to rapidly develop adequate hardness and strength of the shell formed on the match plate to permit it to be stripped off without injury. The first of these results is advantageously accomplished by mixing a hardening accelerator for the liquid refractory composition with the granular refractory which is applied to the surface of the liquid composition on the match plate. The .second of these results is readily achieved by chilling the refractory shell on the match plate to below its freezing temperature. For example, the refractory shell built up on the match plate may be covered with crushed Dry Ice and maintained thus covered until it has become frozen to self-supporting 3 hardness. Other freezing methods may of course also be employed.

Following is a detailed description of a presently preferred embodiment of the invention, and of modifications that can "be made in it with advantage in some cases.

It is first necessary to prepare one or more match plates on which patterns of the desired casting are mounted with suitably located gates, risers, and vents. The patterns on the match plate are not usually of the entire desired casting, but are of portions thereof on one side of a suitable parting plane, as is customary with match plate patterns. The construction of the match plate is in accordance with conventional practice in the foundry art, and is not a part of the invention. The match plate patterns mounted on it may be of any desired material such as wood, plastic composition, or metal. When relatively few castings are to be made wooden patterns and match plates are quite satisfactory, but for making large numbers of castings, and especially where close dimensional tolerances over the entire run is important, metal match plates and patterns are preferred.

It is desirable to mount the match plate for easy handling during production of the half-mold shells. To this end the match plate may be supported on a rack at a convenient working height, and may be mounted on wheels so that it can readily be moved along the rack from one to another of the working stations at which each operation in the production of the shell is carried out. Also, the match plate preferably is mounted on its wheel supports so that it can conveniently be inverted to discharge excess of the refractory material applied to it. The match plate itself preferably is provided with a number of suitably spaced broad-headed ejection pins by which a stripping force can be exerted on the refractory shells produced on the match plate.

The first step in making the refractory mold is to apply a coating of a hardenable refractory composition to the match plate pattern. Any conventional hardenable liquid refractory composition used for making investment casting molds or other types of casting molds may be employed. Such composition basically comprises a slurry of a finely divided refractory dispersed in a liquid vehicle which is capable of setting at room temperature to a nonflowing form. possessing suificient strength to be handled Without injury. Very satisfactory results may be obtained using a refractory composition made by dispersing a finely divided refractory such as finely milled zircon or very fine alumina in an aqueous silica sol vehicle. The composition may in addition include a resinous polymer for contributing increased strength to the composition in the uncured state, a dispersing agent to keep the refractory and polymer constituents thoroughly dispersed in the vehicle, a wetting agent to promote intimate coating of the pattern, and an anti-foaming agent to prevent inclusion of bubbles in the coating of the composition formed on the pattern. The silica sol in such a composition has the property of gelling to a non-flowing self-supporting state when the moisture content of the composition is reduced by evaporation or otherwise, thus causing the composition to harden. With further elimination of water from the composition and upon heating to an elevated temperature the silica precipitated from the sol forms an effective hightemperature bond between the refractory particles.

Another very satisfactory hardenable refractory composition that can be used with advantage is the Well-known dispersion of a finely divided refractory in an aqueous phosphoric acid vehicle. This composition also may contain various additives to improve its characteristics for forming satisfactory coatings on the pattern. Other hardenable refractory compositions are known and may be used successfully.

Since an advantage of the new method is that it enables production of molds from which castings can be made having surfaces as smooth and free from defects as precision castings made by heretofore known methods,

it is desirable to use a refractory composition capable of producing a mold having a surface as. smooth as heretofore known precision casting molds. For this reason it is generally preferable to employ a hardenable refractory composition of the same general character as have been developed for making precision investment castings. The new method may, however, be carried out successfully using any desired hardenable refractory composition.

Preparatory to applying the hardenable refractory composition to the match plate, the exposed surface of the latter and of the patterns on it is preferably coated first with a release agent which prevents the refractory composition from adhering to it. To this same end, a particularly desirable additive to incorporate in the refractory composition is a release agent which reduces the tendency of the composition to adhere to the match plate surface.

The liquid refractory composition is preferably applied by fiowing it over the surface of the match plate. For example, a generous quantity of the composition may be poured from a hand-held vessel on to the match plate and may be distributed over the match plate surface by suitably moving the container about as its contents pour forth. In more highly mechanized installations the composition may be discharged in the form of a stream from a hose held and moved over the match plate, or it may be discharged from a plurality of tubes positioned over the match plate so as to insure proper distribution of the refractory without movement of the tubes. Instead of delivering a solid stream of the composition on to the surface of the match plate, it may be delivered in the form of a spray, and in the latter case the excess of composition applied may be much less than in the former, or there may be no excess. Generally, however, it is most convenient to pour or fiow a substantial excess of the refractory slurrry onto the match plate.

Promptly after applying the refractory slurry, and while it is still readily fluid, the excess is drained from the match plate. This is most conveniently accomplished by inverting the match plate and allowing the excess to run off into a basin or other collecting vessel from which it can be collected for further use. Thereby all but a rather thin coating of the composition, which adheres by the forces of surface tension to the surface of the match plate, is drained off. The match plate then is uprighted again.

While the residual adhering coating of refractory slurry on the match plate surface is still fluid, it is liberally coated with granular refractory particles. This is accomplished by thoroughly sprinkling the fluid coating on the match plate with the dry granular refractory, The dry refractory particles should be relatively coarse, as compared with the fine refractory dispersed in the liquid refractory composition, but not so coarse that they cannot become embedded for half their diameter or more and another small proportion finer than -mesh, for

optimum results. However, the particle size distribution is not critical. It is only necessary that the particles be coarse relative to the refractory dispersed in the liquid refractory composition, but small enough to become em- I bedded through a substantial fraction of their diameter in the coating of such composition adhering to the match plate.

The dry granular refractory particles may be composed of any desired refractory material, such as silica or alumina. Ordinarily it is preferred to use a fused fire clay (grog) crushed to the desired particle size.

As with the liquid composition, any excess of the dry granular refractory particles which does not become embedded in and thus bonded to the liquid refractory coating is removed. Again, this is most conveniently ac complished by inverting the match plate to dump the excess dry granular refractory into a collecting basin, from which it can be taken for further use.

The granule-covered refractory coating is then allowed to harden on the match plate. When it has hardened sufiiciently so as not to be caused to flow by a further application of the liquid refractory composition, another coating of such composition and a further application of dry granular refractory to the new coating is applied in the same manner as described. This procedure is repeated until a refractory shell of the desired thickness has been built up on the match plate. At least three and usually four or five, or even more coats in all of refractory slurry are necessary before the desired final shell thickness is attained.

Since the chief purpose of the dry granular refractory applied to the surface of each coat is to provide for firmly bonding each successive coat to the preceding one. it is not necessary to apply dry granular refractory to the final coat of liquid refractory composition. However, the last coat may be covered with the dry granular refractory, just as the preceding coats, if desired.

The mold cavity in the completed shell is defined only by the first-applied coating of refractory composition.

The subsequent coats serve simply to build up the thickness and hence the strength of the shell. Accordingly, the first coat may with advantage be formed of a liquid composition in which the refractory is specially selected to yield a fine surface (such as finely milled zircon). The subsequent coats then may be formed of a liquid refractory composition in which a different and perhaps somewhat less fine refractory is used, such as a grog of nominally SO-mesh particle size.

It is evident that the time required to build the shell to desired final thickness depends to a considerable extent on the time required for each successive coat of liquid refractory composition to harden. To minimize this time, and so to speed up completion of the shell, it is desirable to mix a hardening accelerator for the liquid refractory composition with the dry granular refractory that is applied to each coat of such composition. Applying a hardening accelerator with the dry granular refractory has the advantage that the coating composition does not tend to harden prematurely while it is being coated on the match plate. But rapid hardening is promoted directly upon applying the dry granular refractory, thus completing the coating for reception of the next successive coat.

The particular nature of the hardening accelerator depends of course on the nature of the liquid coating composition. When the coating composition is based on the use of an aqueous silica sol vehicle which hardens ordinarily by evaporation of water, a good dehydrating agent is a suitable hardening accelerator. For example, Portland cement may be mixed as the hardening accelerator with the dry granular refractory when a silica sol refractory composition is used. Only a small proportion of such accelerator need be mixed with dry granular refractory. Very satisfactory results have been attained with 1% of Portland cement by weight mixed with 99% by weight of grog. In general, the amount of accelerator preferably is in the range from +05% to 3% by weight of the mixture with refractory particles. When the liquid coating composition is based on some other vehicle than silica sol, the hardening agent will be some material which catalyzes or otherwise promotes the hardening reaction.

When the refractory shell has been built up to desired thickness and has hardened, it is stripped from the match plate. It generally happens that the strength of the shell soon after it has hardened on the match plate is inadequate to enable it to be stripped without damage. Various procedures may be employed to accelerate development'of sufficient strength in the hardened shell to permit stripping to occur, or stripping may be deferred until the hardening process advances far enough for adequate strength to be attained. Preferably, however, the shell is prepared for stripping directly after it has hardened, by chilling it to below its freezing temperature. By freezing temperature is meant the temperature at which the residual. liquid component of the vehicle freezes solid. Such chilling of the mold shell develops a high degree of strength and rigidity in the shell, enabling it to be stripped from the match plate cleanly and without injury due to cracking or spalling.

Freezing of the shell may be accomplished in any desired manner. For example, the match plate with shell thereon may be introduced into a refrigerated chamber and kept there until the shell has frozen. A very convenient freezing technique is to cover the shell on the match plate with crushed Dry Ice, and to maintain it thus covered until the shell has frozen. When this technique is employed to freeze shells formed on metallic match plates, the appearance of frost on the bottom surface of the match plate indicates that the refractory shell has been frozen through and is ready to be stripped.

It is preferable, as stated above, especially when large shells must be stripped, for the match plate to be equipped with an adequate number of ejection pins so that upon inserting the match plate in a press, the frozen refractory shell can quickly be stripped off. However, the method used for stripping is not itself a part of the invention. Small mold shells can be stripped by hand, or with hand tools, from the match plate. After stripping the shell, the match plate is inspected for any residual adhering particles of refractory, is cleaned if necessary, and after warming to room temperature is returned for use in forming additional mold shells.

The stripped half-mold shells usually must be heated to an elevated temperature to fully set the refractory composition, and must be assembled together to form the complete mold. The shells may either be assembled first and then heated, or they may be heated first and then assembled, whichever is desired. In either event they are first brought to room temperature and are allowed to remain undisturbed until the refractory composition has developed sufficient strength to permit han dling. This is best accomplished by laying the shell on a supporting surface, on which it is exposed to atmospheric temperature or to a somewhat elevated oven temperature (the latter may accelerate development of sufficient strength for handling of the shell).

If the mold shells are to be heated first to setting temperature and then assembled, they are preferably deposited on a flat supporting surface, resting on the parting plane surface which was formed in contact with the match plate. Thus supported, the shells are introduced into a furnace where they are heated at the desired rate and to the desired temperature for the required period of time to fully set the refractory composition and developing its high temperature bond. The shells are then withdrawn from the furnace and allowed to cool on their flat supporting surfaces, or are allowed to cool on such surfaces in the furnace, whichever is preferred. Itis desirable to keep the shells supported throughout such heating and cooling on their fiat parting plane surfaces in order to minimize any risk that they may become warped.

After cooling, mating half-shells are laid together with their parting plane surfaces in contact, and are aligned as closely as possible. Any necessary cores preferably are laid in place in one of the shells before the mating shells are placed in contact with each other. The shells are then secured together in any desired manner. A convenient and effective manner of doing so is by directly bolting them together, with bolts passing through bolt holes formed at suitable intervals around the periphery of the shells and at other desired locations where the contour of the casting permits. Alternatively the half molds may be secured together by specially designed 7 clamps, or they may be bonded together by means of a high-temperature cement.

If the mold shells are to be assembled first and then heated to set the refractory, the half-shells (after inserting any necessary cores) are laid together, aligned with each other, and secured in the aligned position as described above. The refractory shells, after being warmed from their frozen condition to room temperature and allowed to age for a day or so, possess adequate strength to be handled and assembled together. After assembly the molds are introduced into the furnace in which they are heated to the temperature necessary to fully develop the high temperature bond of the refractory composition. Supports for the assembled shell molds preferably are provided in the furnace to prevent them from becoming warped or otherwise distorted.

It is sometimes desirable to seal the assembled mold to fully close the parting line and to seal around the bolt holes (if the half-molds have been fastened together by bolts). Sealing is particularly desirable when the mold shells have been assembled together prior to heating to fully set the refractory. Sealing is accomplished by applying to the exterior of the mold a coating of a hardenable liquid refractory composition similar to the composition used to build up the shell on the match plate. Such composition may be applied by brushing or spraying, or, if the mold is small enough, by dipping the entire mold in a container of the composition. It is particularly desirable to have the sealing layer bridge the parting line crack and coat the bolts and nuts. Sealing not only closes up small crack and crevices, but provides a bond between the half-shells which prevents them from separating or shifting when the mold is heated and the bolts expand more than the refractory of the shells. After the refractory sealing composition has been applied and has hardened, the sealed mold preferably is heated to a high enough temperature to fully set the sealing refractory.

The assembled mold is employed in the same manner as shell molds generally for making metal castings. The mold is supported in upright position a sand box or other device, and molten metal is poured into it through the gate provided for the purpose. The mold may be preheated preparatory to making the casting, or the hot metal may be poured into the mold while the latter is at or near room temperature, whichever is best and most convenient under the circumstances. If the mold is preheated preparatory to casting, the operation of heating the mold to set the refractory of the shells, or the sealing refractory if such is used, may also serve as the preheating step.

Refractory casting molds may be made in accordance with the invention for use in casting high melting temperature metals and alloys which cannot be cast in metal or other permanent molds. By using available refractory compositions of the character employed for making precision investment castings, the surface quality of the castings made is equal to that of the best precision castings. The dimensional accuracy of castings made in molds produced according to the invention also may be made equal to that of precision castings, except only that dimensional tolerance across the parting plane will usually have to be greater than if the casting were made by precision casting techniques using a variant of the lost wax process. Molds according to the invention may, however, be made much more rapidly, with less labor and less skilled labor, than precision investment casting molds. The new method lends itself very well to high volume production of identical molds, and to a high degree of mechanization, in commercial foundries. It also is adapted to making molds for quite large castings-considerably larger than can be made in commercial production by heretofore known precision casting methods.

Following is an example of the method of this invention: Brass patterns for the casting of a valve body were prepared and mounted on a fiat metal plate to form a match plate pattern. The match plate was provided with a number of broad-headed ejection pins which normally were held by springs with their heads against the match plate, but which could be pressed up from the match plate surface to strip off a shell formed thereon. The pattern, match plate, and ejection pin heads were sprayed with a silicone oil parting agent and baked at 390 F. for one hour.

A first refractory slurry of substantially the following composition was prepared:

Aqueous silica sol (30%) liter 6.075 Polyvinyl acetate copolymer latex (55% solids) do 0.912 Ammonium alginute solution (5%) o 0.759 Wetting agent (phosphatic, 20% s0lution) do 0.243

Release agent (polydimethylsiloxanc) do 0.020 Finely milled zircon minus 325-mesl1) lbs 73 The slurry was prepared by first mixing together the first four ingredients, and then stirring in the zircon and the release agent. This refractory slurry was poured over the match plate, being sure to coat all surfaces thoroughly. The match plate was then inverted to allow all excess composition that did not adhere to the plate and pattern surfaces to drain away. The match plate was then turned righ -side-up, and the still-wet coating of slurry was thoroughly and liberally coated with nominally 70-mesh Alundum. The excess of this dry granular refractory was then dumped off by inverting the match plate, after which it wa again turned right-side-up.

A second refractory slurry of the following composition was prepared:

Aqueous silica sol (30%) liters 6.075 Polyvinyl acetate copolymer latex (55% solids) do 0.912 Ammonium alginatc solution (5%) do 0.759

Wetting agent (phosphatic, 20% solution) do 0.243 Antifoamlng agent (silicone) do 0.004 Finely milled zircon (90% minus 325-mesh) lbs -14 Sintcred fire clay grog (80% minus 20-mesh, plus -mesh) lbs 39.5

This slurry was prepared similary to the first, by first mixing together the first four ingredients and then stirring in the antifoaming agent and the refractories. This second composition was poured over the coating of the first slurry and Alundum on the match plate, after the first slurry coating had hardened to a non-fluid condition. Care was taken to be sure that the first coat was thoroughly covered. The match plate was then inverted until the excess of the second slurry had drained away. Again the match plate was turned right-side-up, and the wet coating of the second refractory composition was liberally and thoroughly sprinkled with the following dry granular mixture:

blgercenltt 0 welgi ibiiiiiit fm ii rfiinls ttb iihii.lffllIjIIIj: 99

Upon hardening of the second coating, threeadditional successive coats of the second refractory slurry and of the fire clay grog-Portland cement mixture were applied to build up a refractory shell about one-quarter inch thick on the match plate. After applying the last coat of refractory slurry and dry granular refractory, the refractory shell on the match plate was thoroughly covered with crushed Dry Ice, and kept thus covered for about 20 minutes until a distinctly visible layer of frost appeared on the bottom surface of the match plate. The match plate was then introduced into a press by which sufficient force was exerted on ejection pins to strip the frozen shell from the match plate. The stripped shell was placed on a flat surface, parting-plane side down, and allowed to warm to room temperature and to dry (2A-hours at room temperature) until it had developed sufficient strength to permit handling Without injury.

The shell thus prepared was carefully inspected for defects, and thoroughly cleaned of flash. A refractory core defining the interior of the valve body was carefully set in place in the shell, and a mating shell that had been similarly prepared was assembled with it. The two half-mold shells were then directly bolted together with steel machine bolts extending through bolt holes that had been molded in the shells on the match plate. The entire shell was then covered with a coat of the second refractory slurry described above, by dipping it in a container of the slurry and allowing excess to drain olf. After this sealing layer of refractory had hardened, the mold was introduced into a furnace at a temperature of about 800 F. The furnace temperature was then raised slowly to 1500 R, and the mold was held in the furnace for eight hours, by which time the refractory shells had developed their full high temperature bond strength. The molds were then taken from the furnace and while still hot were delivered to the casting station and filled with a molten steel alloy at a pouring temperature of 2950 F.

After the metal in the mold had solidified and cooled, the refractory shell molds were broken away from the casting within. The cast valve body possessed a fine smooth surface equivalent to that of a precision investment casting, and conformed closely to the desired dimensions.

We claim:

1. The method of making a refractory casting mold which comprises forming a pair of mating half-mold shells by applying a coating of a hardenable liquid refractory composition to a match plate pattern and embedding in the surface of such coating while it is still fluid a layer of granular refractory particles, hardening the resulting coating on the match plate at about room temperature, and similarly applying successive coatings of hardenable refractory until a refractory shell of substantial thickness has been built up on the match plate, stripping the half-mold shell thus formed from the match plate, and heating the shell to a temperature high enough to fully set the refractory composition.

2-. The method of making a refractory casting mold which comprises forming a pair of mating half-mold shells by applying a coating of a hardenable liquid refractory composition to match plate patterns and embedding in the surface of such coating while it is still fluid a layer of granular refractory particles, hardening the resulting coating on the match plate at about room temperature, and similarly applying successive coatings of hardenable refractory until refractory shells of substantial thickness have been built up on the match plate, stripping the halfmold shells thus formed from the match plate and as sembling them in mating relation, sealing the assembled shell mold by applying to the exterior thereof a coating of hardenable liquid refractory composition and hardening such coating on the mold, and heating the sealed shell mold to a temperature high enough to fully set the refractory composition.

3. The method of making a refractory casting mold which comprises forming a pair of mating half-mold shells by applying a coating of a hardenable liquid refractory composition to a match plate pattern, applying a mixture of a granular refractory and a hardening accelerator for the refractory composition to the surface of such coating while it is still fluid, allowing the coating to harden at about room temperature with the particles of granular refractory embedded in the surface thereof, and similarly applying successive coatings of hardenable refrac tory until refractory shells of substantial thickness have been built up on the match plate, stripping the half-mold shells thus formed from the match plate, and heating the shells to a temperature high enough to fully set the refractory composition.

4. The method of making a refractory casting mold which comprises forming a pair of mating half-mold shells by flowing a hardenable liquid refractory composition over the surface of a match plate pattern, draining excess of such composition from the match plate, covering the residual coating of said composition on the pattern While it is still fluid with a dry granular refractory, removing excess of said granular refractory which does not become embedded in and adherent to the coating, hardening the granule-covered coating at about room temperature, and similarly applying successive coatings of hardenable refractory until refractory shells of substantial thickness have been built up on the match plate, stripping the half-mold shells thus formed from the match plate, and heating the shells to a temperature high enough to fully set the refractory composition.

5. The method of making a refractory casting mold which comprises forming a pair of mating half-mold shells by flowing a hardenable liquid refractory composition over the surface of a match plate pattern, inverting the match plate until the excess composition has drained away, covering the residual coating of such composition on the pattern while it is still fluid with an excess of dry granular refractory, again inverting the match plate to dump off the excess refractory which has not become embedded in and adherent to the coating, hardening the granule-covered coating on the match plate at about room temperature, and similarly applying successive coatings of hardenable refractory until refractory shells of substantial thickness have been built up on the match plate, stripping the half-mold shells thus formed from the match plate, and heating the shells to a temperature high enough to fully set the refractory composition.

6. The method of making a refractory casting mold which comprises forming a pair of mating half-mold shells by flowing a hardenable liquid refractory composition over the surface of a match plate pattern, draining excess of such composition from the match plate, covering the residual coating of said composition on the pattern while it is still fluid with a dry granular refractory, removing excess of said granular refractory which does not become embedded in and adherent to the coating, hardening the granule-covered coating at about room temperature, and similarly applying successive coatings of hardenable refractory until refractory shells of substantial thickness have been built up on the match plate, stripping the half-mold shells thus formed from the match plate and assembling them in mating relation, sealing the assembled shell mold by applying to the exterior thereof a coating of hardenable liquid refractory composition and hardening such coating on the mold, and heating the sealed shell mold to a temperature high enough to fully set the refractory composition.

7. The method of making a refractory casting mold which comprises forming a pair of mating half-mold shells by flowing a hardenable liquid refractory composition over the surface of a match plate pattern, inverting the match plate until the excess composition has drained away, covering the residual coating of such composition on the pattern while it is still fluid with an excess of dry granular refractory mixed with a hardening accelerator for said composition, again inverting the match plate to dump off the excess refractory which has not become embedded in and adherent to the coating, hardening the granule-covered coating on the match plate at about room temperature, and similarly applying successive coatings of hardenable refractory until refractory shells of substantial thickness have been built up on the match plate, stripping the half-mold shells thus formed from the pattern, and heating the shells to a temperature high enough to fully set the refractory composition.

8. The method of making a refractory casting mold which comprises forming a pair of mating half-mold shells by applying a coating of a hardenable liquid refractory composition to a match plate pattern and embedding in the surface of such coating while it is still fluid a layer of granular refractory particles, hardening the resulting coating on the match plate at about room temperature, and similarly applying successive coatings of hardenable refractory until a refractory shell of substantial thickness has been built up on the match plate, chilling the refractory shell on the match plate to below its freezing temperature, stripping the frozen shell from the match plate, and heating the resulting shell to a temlll perature high enough to fully set the refractory composition.

9. The method of making a refractory casting mold which comprises forming a pair of mating halfmold shells by applying a coating of a hardenable liquid refractory composition to a match plate pattern and embedding in the surface of such coating while it is still fluid a layer of dry granular refractory particles, hardening the resulting coating on the match plate at about room temperature, and similarly applying successive coatings of hardenable refractory until a refractory shell of substantial thickness has been built up on the match plate, covering the refractory shell on the match plate with crushed Dry Ice and maintaining it thus covered until it has become frozen, stripping the frozen shell from the match plate, and heating the resulting shell to a temperature high enough to fully set the refractory composition.

10. The method of making a refractory casting mold which comprises forming a pair of mating half-mold shells by applying to a match plate pattern a coating of a hardenable slurry of finely divided refractory dispersed in an aqueous binder vehicle, applying to the surface of such coating while it is still fluid a mixture of granular refractory particles and a hardening accelerator for the refractory composition, allowing the coating to harden at about room temperature to a non-fluid condition with the granules of refractory embedded in the surface thereof, and similarly applying successive coatings until a refractory shell of substantial thickness has been built up on the match plate, chilling the refractory shell on the pattern to below its freezing temperature, stripping the frozen shell from the pattern, and heating the resulting shell to a temperature high enough to fully set the refractory composition.

11. The method of making a refractory casting mold which comprises forming a pair of mating half-mold shells by applying to a match plate pattern a coating of a slurry of a finely divided refractory dispersed in a gelable aqueous silica sol vehicle, applying to the surface of such coating while it is still fluid a mixture of a granular refractory and Portland cement, allowing the coating to gel at about room temperature to a non-fluid condition with the granules of refractory embedded in the surface thereof, and similarly applying successive coatings of refractory slurry until a refractory shell has been built to substantial thickness, then chilling the refractory shell on the match plate to below its freezing temperature, stripping the frozen shells from the match plate, and heating the resulting shell to a temperature high enough to fully set the refractory composition.

12. The method of making a refractory casting mold which comprises forming a pair of mating half-mold shells by flowing over the surface of a match plate pattern a hardenable slurry of finely divided refractory dispersed in a liquid vehicle, draining excess of such slurry from the match plate, covering the residual coating of slurry on the pattern while it is still fluid with a dry granular refractory, dumping off excess of said granular refractory which does not become embedded in and adherent to the coating, allowing the granule-covered coating to harden at about room temperature, and similarly applying successive coatings of refractory slurry until a refractory shell has been built to substantial thi kness on the match plate, then chilling the refractory shell on the match plate to below its freezing temperature, stripping the frozen shells from the match plate, and heating the resulting shell to a temperature high enough to fully set the refractory composition.

13. The method of making a refractory casting mold which comprises forming a pair of mating half-mold shells by flowing over the surface of a match plate pattern a hardenable slurry of finely divided refractory dispersed in a liquid vehicle, inverting the match plate until the excess slurry has drained away, covering the residual coating on the match plate while it is still fluid with an excess of dry granular refractory mixed with a hardening accelerator for the slurry, again inverting the match plate to dump excess refractory which has not become embedded in and adherent to the coating, allowing the granule-covered coating to harden at about room temperature, and similarly applying successive coatings of refractory slurry until a refractory shell has been built to substantial thickness on the match plate, then chilling the refractory shell on the match plate to below its freezing temperature, stripping the frozen shells from the match plate, and heating the resulting shell to a temperature high enough to fully set the refractory composition.

14. The method of making a refractory casting mold which comprises forming a pair of mating half-mold shells by flowing over the surface of a match plate pattern a slurry of finely divided refractory dispersed in a gelable aqueous silica sol vehicle, inverting the match plate until the excess slurry has drained away, covering the residual coating on the match plate while it is still fluid with an excess of dry granular refractory mixed with Portland cement, again inverting the match plate to dump excess refractory which has not become embedded in and adherent to the coating, allowing the granule-covered coating to gel at about room temperature, and similarly applying successive coatings of refractory slurry until a refractory shell has been built to substantial thickness, covering the refractory shell on the match plate with crushed Dry Ice and maintaining it thus covered until it has become frozen, stripping the frozen shell from the metal plate, and heating the resulting shell to a temperature high enough to fully set the refractory composition.

15. The method of making a refractory casting mold which comprises fOnIning a pair of mat-ing half-mold shells by applying a coating of a hardenable liquid refractory composition to match plate patterns and embedding in the surface of such coating while it is still fluid a layer of dry granular refractory particles, hardening the resulting coating on the match plate at about room temperature, and similarly applying. successive coatings of hardenable refractory until refractory shells of substantial thickness have been built up on the pattern, stripping the shells thus formed from the match plate, heating the mold shells sufficiently to dry the refractory composition, directly bolting the dried half-mold shells together in mating relation, sealing the bolted shell mold by applying to the exterior thereof a coating of hardenable liquid refractory composition and hardening such coating on the mold, and heating the sealed shell mold to a temperature high enough fully to set the refractory composition.

References Cited in the file of this patent UNITED STATES PATENTS 2,380,945 Collins et :al Aug. 7, 1945 2,410,954 Sharp Nov. 12, 1946 2,441,695 Pea-gin et al May 18, 1948 2,476,726 Haas July 19, 1949 2,597,370 Peckman May 20, 1952 2,790,722 Kohl et al Apr. 30, 1957 2,792,694 Bartlett et al May 21, 1957 FOREIGN PATENTS 722,816 Great Britain Feb. 2, 1955 

